I’m using a MacBook Pro with an external display. I sometimes need to switch to
only one display — mirroring is useful here. Although I can afford to put the
dock at the bottom in dual-screen mode, it works better on the right if I’m
tight on space. Also when working on the MacBook screen, I can switch to a
higher resolution.

This is where Display Menu comes in. I can pick a display preset (mirroring,
screen resolution, dock position and icon size) — for example, dual screen or
MacBook only, 1440×900 — and get to work quickly.